1. Field of the Invention
The present invention relates to a zoom lens comprising two groups of lenses, a diverging group of lenses and a converging group of lenses which are arranged in sequence starting from the side of an object.
2. Description of the Prior Art
For 35 mm still cameras, it appears that the standard 50 mm lens has been replaced completely by such a two-lens-group type zoom lens having the standard field angle and a zoom ratio equal to about two. Such a type of zoom lens is frequently carried as a common lens mounted on the body of a camera. Since the camera body is being made more compact, the zoom lens must be compact correspondingly. It appears that the zoom lens comprising two groups of positive and negative lenses is most suitable for realizing a zoom lens that is more compact and inexpensive while maintaining its imaging performance at a sufficient level.
Various proposals to miniaturize such a two-lens-group type zoom lens have been made, for example, as disclosed in U.S. Pat. No. 4,190,323. When the following relationship is satisfied: ##EQU1## where f.sub.1 is the focal length in a diverging group of lenses, f.sub.w is the focal length at the wide-angle end and f.sub.T is the focal length at the telephoto end, the total length TLw at the wide-angle end becomes equal to the total length TL.sub.T at the telephoto end. Therefore, the entire lens system can be miniaturized with the optimum balance. At the same time, if the focal length in a convergent group of lenses is decreased, the entire lens system can be made correspondingly compact. When refractive powers are selected for the lenses, however, each group of lenses would be provided with a relatively strong power. This results in increasing the number of lenses in each group and provides a complicated lens system. In addition, since there is the following relationship: EQU f.sub.T =f.sub.1 .multidot..beta..sub.T
where .beta..sub.T is the magnification in the convergent group of lenses as the second group, the convergent group of lenses will be used beyond one-time magnification at the telephoto end. When it is particularly wanted to correct the spherical aberration, the number of lenses in the second group must be increased. The increase of thickness in each of the groups of lenses tends to cancel advantages in minaturization.
On the contrary, if as disclosed in U.S. Pat. No. 4,380,375, the focal length f.sub.1 is increased to provide the following relationship: EQU 0&gt;.beta..sub.T &gt;-1,
that is, for the absolute value of the magnification in the convergent group of lenses as the second group to be smaller than on-time magnification, the number of lenses in each of the lens groups can be decreased to reduce the thickness of each lens group. However, such an arrangement is disadvantageous in that the total length of the telephoto end is decreased while the total length at the wide angle end is substantially increased.